Holographic Memories
After more than 30 years, researchers are
on the verge of using holograms to store data
In memories that are both fast and vast
by Demctri Psaltis and Fai Mok
Optical storage of data has been one of the bright spots in technology over the past 15 years. Compact discs, for example, dominate the market for musical recordings and are now also the standard medium for multimedia releases, which combine text, images and sound. Video games, entire journals, encyclopedias and maps are among the multimedia products available on CDs to users of personal computers.
Without a doubt, optical memories store huge amounts of digitized information inexpensively and conveniently. A compact disc can hold about 640 million bytes—enough for an hour and a quarter of high-fidelity music or more than 300,000 pages of double-spaced, typewritten text. All indications are, however, that these large memories have stimulated demand for even more capacious and cheaper media. Executives in the entertainment industry would like to put one or more motion pictures on a single optical disk the same size as a CD, and so great are the data storage needs of some hospitals, law firms, government agencies and libraries that they have turned to so-called jukeboxes that have-robotic arms to access any one of hundreds of disks.
Engineers have responded by trying to wTing the most out of CD systems. Some are working on semiconductor lasers with shorter wavelengths (in effect, these will be finer styli that permit closer spacing of bits on a CD). Others are investigating techniques of data compression and "super-resolution" that also allow higher density (the latter at the expense of increased background noise). Another promising development has been multiple-level CDs, in which two or more data-containing tracks are stacked and read by an optical system that can focus on one level at a time. Such schemes are expected to push the capacity of CDs into the tens of billions of bytes within five years or so.
But to pack a CD-size disk with much more data—hundreds of billions of bytes, say—will require a fundamentally different approach: holography. The idea dates back to 1963, when Pieter J. van Heerden of Polaroid first proposed using the method to store data in three dimensions.
Holographic memories, it is now believed, could conceivably store hundreds of billions of bytes of data, transfer tiu::" at ?> rnto of a billion or more bits per second and select a randomly chosen data element in 100 microseconds or less. No other memory technology that offers all three of these advantages is as close to commercialization— a fact that has compelled such large companies as Rockwell, IBM and GTE in the past two years to launch or expand efforts to develop holographic memories.
Initially, the expense and novelty of the technology will probably confine it to a handful of specialized applications demanding extraordinary capacity and speed. Such uses are already attempting to carve out little niches—one recently offered product holographically stores the fingerprints of those entitled to enter a restricted area, permitting access when a matching finger is placed on a glass plate. If in meeting such needs the technology matures and becomes less expensive, it might supersede the optical disk as a high-capacity digital storage medium for general-purpose computing.
The main advantages of holographic storage- high density and speed—come
HOLOGRAPHIC MEMORY stores data in a crystal of lithium niobate not much larger than a sugar cube {foreground). The hologram is created in the crystal by the meeting of a reference laser beam, shown thick and bright in this photograph, and a signal beam, fainter and thinner, which contains the data.
from three-dimensional recording and from the simultaneous readout of an entire page of data at one time. Uniquely, holographic memories store each bit as an interference pattern throughout the entire volume of the medium.